The MT has bakodesh (singular) for IN hAGIOIS. This leads a lot of
translators and some commentators to ignore the LXX reading which I
think is a mistake in this instance.

These two lines in the LXX give evidence that the Hebrew vorlage of the
LXX had a parallel structure here. So rather than "majestic in holiness"
(NIV, U. Cassuto) we might translate it "Majestic among the holy ones"
(Everett Fox).

U. Cassuto ignores the LXX but you know he has seen it because he makes
the comment that "majestic in holiness"corresponds to "majestic in
power" in Ex 15:6. This comment would be unnecessary if he were not
finding an excuse to overlook the reading in the LXX. U. Cassuto's
motive appears to be that he wants to find nothing in Torah which could
be even remotely understood as polytheistic. This is my speculation and
nothing more. However reading his commentary on Genesis 1-2 has led me
to wonder about his treatment of Ex 15:11b.

Everett Fox (The Five Books of Moses) has adopted the LXX reading and
translated it as "Majestic among the holy ones." If we see Ex. 15:11a
and 15:11b as a form of parallelism then I think that the LXX reading is
probably the correct one and I would go with E. Fox on this and not U.
Cassuto.